The invention relates to cutter assemblies and cutting elements for drag-type rotary drill bits for use in drilling or coring holes in subsurface formations.
Such rotary drill bits are of the kind comprising a bit body having a shank for connection to a drill string, a plurality of cutter assemblies mounted at the surface of the bit body, and a passage in the bit body for supplying drilling fluid to the surface of the bit for cleaning and/or cooling the cutters. Each cutter assembly comprises a preform cutting element mounted on the bit body or, more usually, on a carrier which is then mounted on the bit body.
One common form of preform cutting element comprises a tablet, for example circular, having a thin superhard cutting table of polycrystalline diamond bonded to a thicker substrate of a material which is less hard than the polycrystalline diamond. The preform cutting element is then mounted on the carrier, for example by a process known as "LS bonding".
The carrier, which is usually generally cylindrical in shape, is received in a socket in the surface of the bit body. The bit body itself may be machined from metal, usually steel, or may be moulded using a powder metallurgy process.
In known cutter assemblies of this type it has been usual for the substrate of the cutting element and the carrier itself to be formed from cemented tungsten carbide which has characteristics which render it particularly suitable for this purpose. Thus, it exhibits high rigidity, high resistance to the erosion to which such carriers are subject in use, and hot strength. Also, the coefficient of expansion of tungsten carbide is sufficiently close to the coefficient of expansion of polycrystalline diamond to reduce the residual stresses which can occur when the two materials are bonded together.
However, some of the other characteristics of cemented tungsten carbide have certain disadvantages. For example, cemented tungsten carbide has low toughness (i.e. it is comparatively brittle) and this can lead to failure of such cutter assemblies in use, as a result of impact forces on the assembly. Also after prolonged use of a rotary drill bit, a large wear flat develops on the substrate and carrier rearwardly of the diamond table and bears on the formation being drilled. Due to the high abrasion resistance of tungsten carbide, this leads to high heat generation due to friction, with consequent overheating and premature failure of the polycrystalline diamond table of the preform cutting element. The combination of low toughness and high heat generation can also cause heat checking of the tungsten carbide carrier material with resultant premature failure of the bit. "Heat checking" is a term in the art which refers to craze cracking of the wear flat which develops on the carrier due to abrasive heating with intermittent quenching by the drilling fluid.
Accordingly, there may be advantages in using for the substrate and/or carrier a material which is less abrasion resistant than the tungsten carbide normally employed. For example, there has been proposed in our British Patent Specification No. 2216577 the use, for the carrier of such a cutter assembly, of a material containing at least a proportion of tungsten metal. Our British Patent Specification No. 2228031 discloses the use in a cutting element of a substrate containing at least a proportion of tungsten metal.
The presence of tungsten metal in the carrier or substrate which, as described in the earlier applications, may be an addition to the tungsten carbide or may entirely replace the tungsten carbide, has the effect of reducing the abrasion resistance of the carrier or substrate so as to reduce the additional heat generated by rubbing of the wear flat on the formation being drilled. In addition the tungsten-containing material may be even stronger than cemented tungsten carbide in resisting the cantilever bending and shear forces to which cutter assemblies may be subject in use.
However, it is desirable that the abrasion resistance of the carrier and substrate should be reduced without also significantly reducing its erosion resistance. In use the cutter assemblies are subjected to the substantial eroding effect of drilling fluid flowing over the cutter assemblies continuously during drilling. Existing cutter assemblies where the carrier and substrate comprise cemented tungsten carbide have considerable resistance to such erosion. The use of tungsten metal (or other material of lower abrasion resistance than tungsten carbide) in the carrier or substrate tends however to reduce the erosion resistance and this may limit the extent to which such materials may be used.